Mammalian mitochondria contain 55S ribosomes of unusual composition and physical-chemical properties. The goal of the proposed research is the elucidation of the structure, composition, function, and biosynthesis of mammalian mitochondrial ribosomes. Mitochondrial ribsomes produced on a large scale from bovine liver are being characterized structurally and functionally to provide a model system for the study of mammalian mitochondrial ribosomes. The individual r-proteins will be characterized with respect to their MW, isoelectric point, stoichiometry, amino acid composition, N-terminal amino acid, tryptic peptide fingerprint, and N-terminal amino acid sequence. Approaches to examine the structural organization of mitochondrial r-proteins will include: the accessibility of r-proteins in monoribosomes and subribosomal particles to surface probes and the identification of r-proteins in "core" particles and fragments of the subribosomal particles. Approaches to identify individual r-proteins within defined functional domains will include the functional reconstitution of protein deficient subribosomal particles and labelling with site-specific affinity probes. The interspecific variation of mammalian mitochondrial r-proteins will be analyzed to assess their rapid evolutionary divergence. Experiments will also be performed to ascertain which of the mitochondrial r-proteins are made in mitochondria and to characterize the cytoplasmic ribosomes involved in the synthesis of mitoribosomal proteins.